Quite often health services policy must be determined based on naturally occurring experiments in time. Because a true experimental design could not be conducted, quasi-experimental designs must be used in order to be able to draw valid conclusions. However, among the many quasi-experimental design which are available differences exist as to the validity of conclusions which can be made; criteria used to evaluate the approporiateness of each design must be based on considerations of the internal and external validity of each. The recently funded study of the effects of cost-sharing on the beneficiaries of the United Mine Workers (UMW) health plan provides an excellent example of this problem. Because of the importance of cost-sharing in relation to proposals for National Health Insurance, it is crucial that the most powerful and efficient methods be used to analyze this data so that valid conclusions can be drawn which can help serve as a guide to determining policy related to National Health Insurance. Since recent studies of the effect of cost-sharing have suffered from several methodological flaws, the question of which quasi-experimental design to use on the UMW data and others like it in the future needs further consideration. The purpose of this proposal is to evaluate which quasi-experimental design should be used to provide valid conclusions in a naturally occurring experiment of this type. Data provided by the UMW will be used to compare these methodologies. The main focus of this comparison will be the use of a quasi-experimental design known as the "time reversed crossover design" which incorporates many of the powerful features of the crossover design. Three other quasi-experimental designs will also be considered and all four designs will be applied to a selected subset of variables in the UMW data set. Each design will be compared using criteria based on the validity of conclusions which can be drawn; the results of each as applied to the UMW data will also be compared to determine if different conclusions would be drawn from each. The major emphasis of these comparisons will be the differences in these methods and the possibility of different policy decisions being made as a result of not using the most optimal quasi-experimental design available.